Why do the objects imported to libGDX look white?
Had you checked in blender's cycles render? If not, you probably not handle the material correctly.

In my case, this defection comes from the material arrangement as a result of my unfamiliar with blender. I think it's the material should loaded by gdx until i compared the results from the same java code with the obj file used by xoppa's tutorial. see the tutorial:

https://xoppa.github.io/blog/loading-models-using-libgdx/

The ship's material is showing.

You'd better check it.

Another way to check the material is exported correctly is open and search your obj or g3dj files.

In obj file's 'f' section, the vertex's format is position/uv/norm, see

https://people.cs.clemson.edu/~dhouse/courses/405/docs/brief-obj-file-format.html
My case is "3//0.3, ...", so there the problem is picture not mapped to vertex.

If you open the g3dj file, the lack of UV mapping will makes fbx-conv converting empty UV mapping, like:
    {
        "version": [  0,   1], 
        "id": "", 
        "meshes": [
            {
                "attributes": ["POSITION", "NORMAL"], 
                "vertices": [ some-6-dimesional-vectors ...
This vertices lacking UV data. The "attributes" should actually 3 attribures:

    ["POSITION", "NORMAL", "TEXCOORD0"]

And the vertices should be 8 elements.
To unwrap and uv mapping, see blender manual:


https://docs.blender.org/manual/en/latest/editors/uv_image/uv/editing/unwrapping/mapping_types.html#unwrap

Hope this will be helpful although years latter.